1. Field of the Invention
The invention relates to instant of ignitions having exhaust gas recirculation. In particular, the present invention relates to the determination or regulation of an exhaust gas recirculation rate in an instant of ignition.
2. Description of the Prior Art
In lean-operated engines, for instance in engines with a stratified charge, there is an air excess in the combustion chamber of each cylinder. Because of the lean mode of operation, engines can be operated with greater efficiency. However, a lean mode of operation leads to increased emissions of nitrogen oxide, which as a rule exceeds the applicable limits for nitrogen oxide. To reduce nitrogen oxide emissions, an external exhaust gas recirculation is therefore provided, in which some of the exhaust gas produced in the engine is returned to the intake tube of the engine in order to be introduced there into the cylinders of the engine again.
In optimizing nitrogen oxide emissions, an exhaust gas quantity determined by an operating point has to be returned. As a rule, this quantity is represented by what is called an exhaust gas recirculation rate, which indicates the proportion of exhaust gas in the fresh air delivered. Metering the returned exhaust gas quantity is typically done in pilot-controlled fashion, taking into account a location feedback of the inlet valves used for the exhaust gas recirculation.
Since the exhaust gas recirculation rate cannot be measured directly, until now it has had to be calculated using a suitable model. This modeled exhaust gas recirculation rate is then used for regulating the exhaust gas recirculation rate to a set-point value defined by an operating point. Calculating the quantity of exhaust gas actually returned to the cylinder is done on the basis of a pressure sensor and air flow rate-based balancing of the inflowing and outflowing quantities of air and exhaust gas. In the fully unthrottled mode of engine operation, which is preferentially assumed in a lean mode of operation, the erroneous variations of this modeling of the actually returned exhaust gas quantity, or the actual exhaust gas recirculation rate, rise sharply.
Particularly in the lean mode of the engine, the combustion properties worsen drastically with an increasing exhaust gas recirculation rate, and even at exhaust gas recirculation rates elevated only slightly above the predetermined set-point value lead to combustion misfires. If the actually delivered exhaust gas recirculation rates drop below the exhaust gas recirculation rates that are optimal for attaining minimum nitrogen oxide expulsion, then the nitrogen oxide emissions rise disproportionately, which leads to a marked disadvantage in terms of fuel consumption, because of the necessity of more-frequent regeneration of the nitrogen oxide storage-type catalytic converters employed.